U.S. President Barack Obama said on Thursday that investigators do not yet know why two suspects killed 14 people and wounded 17 others in a mass shooting in Southern California, but vowed that the FBI and law enforcement would “get to the bottom of this.”

“It is possible that this was terrorist-related. But we don’t know. It is also possible that this was workplace-related,” said Obama, who ordered flags flown at half-staff after the tragedy.

The San Bernardino shooting is the latest in a long series of U.S. mass shootings during Obama’s seven years in office, and is the deadliest since the December 2012 shooting at Sandy Hook elementary school in Newtown, Connecticut, which he has said was his toughest day as president.

Speaking from the Oval Office after meeting with his national security advisers, Obama expressed sympathy for the victims and said the United States needs to pass laws to “make it a little harder” for people seeking to do harm to get access to weapons.

But Obama took a less angry tone than he has used after other recent mass shootings, and sought to reassure Americans who are nervous after attacks in Paris by Islamic State militants last month.

He said the Federal Bureau of Imvestigation would take the lead in the investigation and would do a “large number of interviews” and pore over “social media and electronic information.” But Obama said the shooters may have had “mixed motives” which could make the investigation more complicated, and warned it could take “some time” to reach conclusions.

“We do know that two individuals who were killed were equipped with weapons and appeared to have access to additional weaponry in their home. But we don’t know why they did it,” he said.

“We don’t know at this point the extent of their plans. We do not know their motivations,” he said.